CN112513312B

CN112513312B - Method for manufacturing hot-dip galvanized steel sheet and method for manufacturing alloyed hot-dip galvanized steel sheet

Info

Publication number: CN112513312B
Application number: CN201980050380.0A
Authority: CN
Inventors: 古川直人; 小西刚嗣; 福原拓朗; 饭田真一郎
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-07-30
Filing date: 2019-07-29
Publication date: 2023-03-31
Anticipated expiration: 2039-07-29
Also published as: JP7028324B2; JPWO2020027017A1; KR20210032507A; KR102465586B1; TW202012654A; CN112513312A; WO2020027017A1

Abstract

The invention provides a method for manufacturing a hot-dip galvanized steel sheet, which can inhibit the generation of zinc dross defects even though bottom dross is generated in the hot-dip galvanizing treatment. The manufacturing method of the present invention includes: a coarse bottom dross generation step of adjusting the free Al concentration C in the hot dip galvanizing bath so as to satisfy the formula (1) _Al And a bath temperature T, thereby generating coarse bottom dross in the hot dip galvanizing bath; and a hot dip galvanizing treatment step of adjusting the free Al concentration C of a hot dip galvanizing bath containing coarse bottom dross so as to satisfy the formula (2) _Al And performing hot-dip galvanizing treatment at the bath temperature T to form a hot-dip galvanized layer on the steel sheet. 466.15 XC _Al +385.14≤T≤577.24×C _Al +382.49 (1)390.91×C _Al + 414.20. Ltoreq. T.ltoreq.485.00 (2) < CHEM > C in formula (1) and formula (2) _Al "free Al concentration C in hot dip galvanizing bath _Al (mass%).

Description

Method for manufacturing hot-dip galvanized steel sheet and method for manufacturing alloyed hot-dip galvanized steel sheet

Technical Field

The present invention relates to a method for producing a hot-dip galvanized steel sheet and a method for producing an alloyed hot-dip galvanized steel sheet.

Background

Hot-dip galvanized steel sheets (hereinafter, also referred to as GI) and alloyed hot-dip galvanized steel sheets (hereinafter, also referred to as GA) are produced by the following production steps. First, a steel sheet (base steel sheet) to be subjected to hot dip galvanizing is prepared. The base steel sheet may be a hot-rolled steel sheet or a cold-rolled steel sheet. The prepared base steel sheet (the hot-rolled steel sheet or the cold-rolled steel sheet) is immersed in a hot-dip galvanizing bath to be subjected to hot-dip galvanizing treatment, thereby producing a hot-dip galvanized steel sheet. When producing the galvannealed steel sheet, the galvannealed steel sheet is further subjected to heat treatment in an alloying furnace to produce a galvannealed steel sheet.

The hot-dip galvanizing treatment performed in the production process of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet is described in detail below. A hot-dip galvanizing facility for a hot-dip galvanizing process includes: the hot dip galvanizing system comprises a molten zinc pot containing a hot dip galvanizing bath, an immersion roller arranged in the hot dip galvanizing bath, and a gas wiping device.

In the hot dip galvanizing treatment, for example, the steel sheet subjected to the annealing treatment is immersed in a hot dip galvanizing bath. Then, the traveling direction of the steel sheet is shifted upward by the submerged rolls disposed in the hot dip galvanizing bath, and the steel sheet is pulled up from the hot dip galvanizing bath. The wiping gas is blown from the gas wiping device onto the surface of the steel sheet that is pulled up and travels upward. The wiping gas scraped off the remaining molten zinc to adjust the amount of plating deposited on the steel sheet surface. The hot dip galvanizing treatment is performed by the above method. In the case of manufacturing an alloyed hot-dip galvanized steel sheet, the steel sheet with the plating adhesion amount adjusted is further charged into an alloying furnace and subjected to alloying treatment.

In the hot dip galvanizing treatment, fe is eluted from the steel sheet immersed in the hot dip galvanizing bath into the hot dip galvanizing bath. Fe eluted from the steel sheet into the hot dip galvanizing bath reacts with Al and Zn present in the hot dip galvanizing bath to form an intermetallic compound called zinc dross. For zinc dross, there is top dross and bottom dross. The top dross is an intermetallic compound having a specific gravity lower than that of the hot dip galvanizing bath, and is zinc dross floating on the liquid surface of the hot dip galvanizing bath. The bottom slag is an intermetallic compound having a specific gravity heavier than that of the hot dip galvanizing bath, and is zinc slag deposited on the bottom of the molten zinc pot.

In the hot dip galvanizing treatment, wake up current is generated as the steel sheet travels in a hot dip galvanizing bath. The accompanying current means that a water flow is generated in the hot dip galvanizing bath along with the advance of the steel sheet. As described above, the top dross floats on the surface of the hot dip galvanizing bath and is therefore hardly affected by the accompanying current. On the other hand, the bottom slag is deposited on the bottom of the molten zinc pot. Therefore, the bottom slag sometimes winds up from the bottom of the accumulated molten zinc pot along with the wake. In this case, the bottom dross may float in the hot dip galvanizing bath. Such floating bottom dross may adhere to the surface of the steel sheet in the hot dip galvanizing process.

The bottom dross adhering to the surface of the steel sheet may form a spot-like defect on the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet. In the present specification, the surface defect caused by such bottom dross is referred to as "dross defect". The dross defect reduces the appearance or corrosion resistance of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet. Therefore, it is preferable that the generation of the zinciferous slag defects be suppressed.

Japanese patent application laid-open No. 11-350096 (patent document 1) and Japanese patent application laid-open No. 11-350097 (patent document 2) propose techniques for suppressing the occurrence of a zinc dross defect.

In patent document 1, in the method for producing a galvannealed steel sheet, the molten zinc bath temperature is T (° c), and the boundary Al concentration defined by Cz = -0.015 × T +0.76 is Cz (wt%). At this time, the molten zinc bath temperature T is brought within a range of 435 to 500 ℃ and the Al concentration in the bath is kept within a range of Cz. + -. 0.01 wt%. In patent document 1, at δ ₁ The boundary between the phase and the zeta phase is subjected to hot dip galvanizing treatment. In patent document 2, in the method of manufacturing an alloyed hot-dip galvanized steel sheet, the Al concentration in the bath is maintained in the range of 0.15 ± 0.01 wt%. In patent document 2, the slag is separated from the slag by a ₁ The hot dip galvanizing treatment is performed at the phase boundary.

Documents of the prior art

Patent literature

Patent document 1: japanese patent laid-open publication No. 11-350096

Patent document 2: japanese patent laid-open publication No. 11-350097

Non-patent document

Non-patent document 1: practical Applications of Phase diagnostics in Continuous galvanization, nai-Yong Tang, journal of Phase Equilibria and Diffusion Vol.27No.5,2006

Disclosure of Invention

Problems to be solved by the invention

As described in

patent documents

1 and 2, it is known that when the Al concentration in the hot dip galvanizing bath is increased, most of the zinc dross becomes top dross instead of bottom dross. The top dross floats on the surface of the hot dip galvanizing bath. Therefore, it is easier to remove the top dross from the hot dip galvanizing bath than to remove the bottom dross from the hot dip galvanizing bath. Therefore, in the conventional hot dip galvanizing treatment, the following method may be used: the Al content in the hot dip galvanizing bath is increased, so that the zinc dross in the hot dip galvanizing bath floats on the liquid surface of the hot dip galvanizing bath as top dross and is removed, and the generation of zinc dross defects is inhibited. Therefore, in the present specification, the operation of producing the top dross as the zinc dross is referred to as a top dross operation.

The zinc dross defect can be inhibited in the top dross operation. However, when the Al concentration in the hot dip galvanizing bath is increased, the hot dip galvanized layer is difficult to be alloyed in the alloying treatment. Therefore, in order to promote alloying, it is particularly preferable to suppress the Al concentration in the hot-dip galvanizing bath. When the slag tapping work is performed, the Al concentration in the hot dip galvanizing bath inevitably increases.

In the present specification, an operation of suppressing the Al concentration in the hot dip galvanizing bath and generating bottom dross as zinc dross is referred to as bottom dross operation. In the case of the bottom dross working, the alloying can be promoted because the concentration of free Al in the hot dip galvanizing bath is suppressed. However, in the case of bottom dross operation, a method capable of suppressing the formation of bottom dross and causing a zinc dross defect is required.

An object of the present invention is to provide a hot-dip galvanized steel sheet and a method for producing an alloyed hot-dip galvanized steel sheet, in which the occurrence of a zinc dross defect can be suppressed even when bottom dross is generated in a hot-dip galvanizing process.

ForMeans for solving the problems

The method for manufacturing a hot-dip galvanized steel sheet according to the present invention includes:

a coarse bottom slag generation step of adjusting the free Al concentration C in the hot dip galvanizing bath so as to satisfy the formula (1) _Al (mass%) and a bath temperature T (DEG C) to generate coarse bottom dross having a particle size of 300 [ mu ] m or more in the hot dip galvanizing bath; and

a hot dip galvanizing treatment step of adjusting the free Al concentration C of the hot dip galvanizing bath after the coarse bottom dross generation step so as to satisfy formula (2) _Al And the bath temperature T, using the free Al concentration C _Al And performing a hot-dip galvanizing treatment using the hot-dip galvanizing bath having the bath temperature T satisfying the formula (2), thereby forming a hot-dip galvanized layer on the steel sheet.

466.15×C _Al +385.14≤T≤577.24×C _Al +382.49 (1)

390.91×C _Al +414.20≤T≤485.00 (2)

The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present invention includes:

a step of manufacturing a hot-dip galvanized steel sheet by performing the method for manufacturing a hot-dip galvanized steel sheet; and

and an alloying treatment step of alloying the hot-dip galvanized steel sheet.

ADVANTAGEOUS EFFECTS OF INVENTION

The method for producing a hot-dip galvanized steel sheet and the method for producing an alloyed hot-dip galvanized steel sheet according to the present invention can suppress the occurrence of a dross defect even if a bottom dross is generated in the hot-dip galvanizing treatment.

Drawings

FIG. 1 is a graph showing the relationship between a dross formation phase in a hot dip galvanizing bath and the bath temperature T (. Degree. C.) and the free Al concentration C _Al A meta-stable state diagram is compiled.

Fig. 2 is a functional block diagram showing the overall structure of a hot-dip galvanizing line apparatus for manufacturing an alloyed hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet.

Fig. 3 is a side view of the hot-dip galvanizing apparatus in fig. 2.

Fig. 4 is a side view of a hot-dip galvanizing apparatus having a structure different from that of fig. 3.

Fig. 5 is a side view of a hot dip galvanizing apparatus having a structure different from that of fig. 3 and 4.

Fig. 6 is a functional block diagram showing the overall configuration of hot-dip galvanizing line equipment having a configuration different from that of fig. 2.

FIG. 7 is a schematic view for explaining a method of measuring the particle size of the bottom ash.

FIG. 8 is a photomicrograph showing the form of bottom dross formed in the hot dip galvanizing bath after 10 days of bath preparation in examples 1 and 2.

FIG. 9 is a graph showing the relationship between the particle size and the number of zinc dross under the respective production conditions of example 5.

Detailed Description

[ causes of Zinc dross Defect ]

First, the present inventors have studied about the zinc dross which causes the defect of the zinc dross when the bottom dross operation is performed. Regarding the zinc dross defect, zinc dross generated in the hot dip galvanizing treatment is a cause of the defect. In the past, it has been reported that the following types of zinc dross are generated in the hot dip galvanizing treatment.

(A) Slag jacking

(B)δ ₁ Phase zinc slag

(C)Γ ₁ Phase zinc slag

(D) Zeta phase zinc dross

As described above, the top dross has a lower specific gravity than the hot dip galvanizing bath. Therefore, the top dross tends to float on the surface of the hot dip galvanizing bath. The chemical composition of the top slag consists of, in mass%, 45% of Al, 38% of Fe and 17% of Zn. Since the top dross floats on the liquid surface of the hot dip galvanizing bath, it is easily removed from the hot dip galvanizing bath. Therefore, when the slag-pushing operation is performed, the slag-pushing operation can be removed, so that the zinc slag defect can be effectively suppressed.

On the other hand, δ ₁ Phase zinc dross, gamma ₁ The phase dross and the ζ -phase dross are referred to as bottom dross. The specific gravity of the bottom slag is heavier than that of the hot dip galvanizing bath. Therefore, the bottom dross is likely to accumulate at the bottom of the molten zinc pot storing the hot-dip galvanizing bath. In the past, it was considered that the present invention was carried out in the followingThese bottom slags become the cause of zinc dross defects during slag operation.

Where, delta ₁ The crystal structure of the phase zinc slag is hexagonal. Delta ₁ The chemical composition of the phase zinc dross consists of, in mass%, 1% or less of Al, 9% or more of Fe, and 90% or more of Zn. Gamma-shaped ₁ The crystal structure of the phase zinc slag is face-centered cubic crystal. Gamma-shaped ₁ The chemical composition of the phase zinc dross consists of 20% by mass of Fe and about 80% by mass of Zn. The crystal structure of the zeta-phase zinc dross is monoclinic. The chemical composition of the ζ phase is composed of, by mass%, 1% or less of Al, about 6% of Fe, and about 94% of Zn.

In the past, there have been many reports that the main cause of the zinciferous slag defect in the bottom slag was δ ₁ And (4) phase zinc slag. The

above patent documents

1 and 2 also consider δ ₁ The phase zinc dross is one of the causes of the defects of the zinc dross. Therefore, the present inventors originally thought that δ ₁ The phase zinc dross is the main cause of zinc dross defects, and investigation and research are carried out. However, even in the hot dip galvanizing treatment, δ is suppressed ₁ Although phase dross is generated, dross defects may still occur on the surfaces of the alloyed hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet.

Therefore, the present inventors considered that the cause of the zinciferous slag defect may not be δ ₁ Phase zinc dross, but other zinc dross. Therefore, the present inventors performed a bottom dross working using an alloyed hot-dip galvanized steel sheet in which a dross defect occurred, and newly analyzed the composition and crystal structure of the dross defect portion. The present inventors have also newly analyzed the kind of zinc dross generated in the hot dip galvanizing bath in the bottom dross operation. As a result, the present inventors have obtained the following findings regarding the defects in the zinc dross, which are different from the conventional findings.

First, the chemical composition of a dross defect portion on the surface of an alloyed hot-dip galvanized steel sheet was analyzed by using EPMA (Electron Probe Micro Analyzer: electron Probe microanalyzer). Further, the crystal structure of the zinciferous slag defect portion was analyzed using TEM (Transmission Electron Microscope). As a result, the chemical composition of the defective portion of the zinciferous slag consisted of 2% by mass of Al, 8% by mass of Fe and 90% by mass of Zn, and the crystal structure was face-centered cubic.

Delta, which has been considered to be a cause of the conventional zinciferous slag defect ₁ The chemical composition of the phase dross (in mass%, 1% or less of Al, 9% or more of Fe, and 90% or more of Zn) is similar to that of the above-described dross defect portion. However, delta ₁ The crystal structure of the phase dross is hexagonal, rather than face-centered cubic, as identified in the dross defect fraction. Therefore, the present inventors considered that δ, which has been conventionally considered to be a main cause of the zinciferous slag defect, was δ ₁ Phase dross is not actually the main cause of dross defects.

Therefore, the present inventors identified the dross which causes the dross defect. In the above-mentioned zinc dross of (B) to (D), F ₁ The crystal structure of the phase dross is a face-centered cubic crystal similar to that of the dross defective portion, but the chemical composition (20% by mass of Fe and 80% by mass of Zn) is significantly different from that of the dross defective portion. The chemical composition of the ζ -phase dross (1% or less of Al, about 6% of Fe, and about 94% of Zn by mass%) is different from the chemical composition of the dross defect portion, and the crystal structure (monoclinic crystal) is also different from the crystal structure (face-centered cubic crystal) of the dross defect portion.

Based on the above results, the present inventors considered that the dross defects were not caused by the above-described dross (B) to (D). The present inventors also considered that the dross defect may be caused by other types of dross than the above-described ones (B) to (D).

Therefore, the present inventors further analyzed the bottom dross in the hot dip galvanizing bath. The above EPMA and TEM were used for the analysis of the bottom dross. As a result, the present inventors newly found that Γ is present as a bottom dross generated in a hot dip galvanizing bath ₂ And (4) phase zinc slag.

Γ ₂ The chemical composition of the phase dross consists of, in mass%, 2% of Al, 8% of Fe and 90% of Zn, and corresponds to the chemical composition of the above-analyzed dross defect portion. And, Γ ₂ The crystal structure of the phase zinc dross is face-centered cubic crystal, which is consistent with the crystal structure of the defective part of the zinc dross. Therefore, the temperature of the molten metal is controlled,the inventors believe that ₂ The phase dross may be the main cause of the dross defect. And, Γ ₂ The specific gravity of the phase zinc dross is larger than that of the hot dip galvanizing bath, so that the gamma ray is ₂ The phase zinc slag belongs to bottom slag which can be accumulated at the bottom of a molten zinc pot.

The inventors further aimed at ₂ The phase zinciferous slag and the zinciferous slags of (B) to (D) were examined. As a result, the zinc dross defect is caused by hard zinc dross, and the soft zinc dross defect is not easy to form. As a result of further investigation, the inventors of the present invention have found that the above-mentioned zinc dross and gamma of (B) to (D) ₂ In the phase zinc dross, gamma-ray ₂ The phase dross is the hardest dross.

Based on the above results, the present inventors considered that the main cause of the dross defect generated on the surfaces of the galvannealed steel sheet and the hot-dip galvanized steel sheet to be subjected to the hot-dip galvanizing treatment is not δ ₁ Phase zinc dross, rather, gamma ₂ And (5) phase zinc slag. Further, the present inventors have found that: although the zinc dross classified as bottom dross is F ₂ Slag of zinc phase, delta ₁ Phase zinc dross, zeta phase zinc dross and gamma-type ₁ Any one of the phase zinc dross, but substantially no Γ is present in the hot dip galvanizing bath ₁ And (4) phase zinc slag.

Therefore, the present inventors have further studied about the dross defect when the bottom dross operation is performed while suppressing the free Al concentration in the hot dip galvanizing bath. The accompanying current caused by the passage of the steel sheet through the hot dip galvanizing bath causes a part of the bottom slag deposited on the bottom of the molten zinc pot to be curled up. Then, the rolled bottom dross adheres to the steel sheet. In this case, the zinc dross defect is easily generated.

Here, the present inventors paid attention to the size of the bottom ash and conducted studies. As a result, the present inventors have obtained the following findings. The bottom dross with a particle size of less than 100 μm is defined as fine bottom dross. The fine bottom dross may be carried up from the bottom of the molten zinc bath into the hot dip galvanizing bath with the accompanying current. However, even if the fine bottom dross is rolled up and adheres to the steel sheet, the bottom dross is not likely to become a dross defect because of its small size. On the other hand, the bottom dross with a particle size of more than 300 μm was defined as coarse bottom dross. The mass of the coarse bottom slag is heavy. Therefore, the coarse bottom dross is less likely to be turned up by the accompanying flow, and is less likely to adhere to the steel sheet. From the results of the above studies, the present inventors have found that the bottom dross, which is a cause of the zinciferous dross defect, is a bottom dross (hereinafter, referred to as a middle-sized bottom dross) having a particle size of 100 to 300. Mu.m.

Therefore, the present inventors considered that, even when the bottom dross operation is performed, if the formation of the middle-size bottom dross can be suppressed during the hot-dip galvanizing treatment (hereinafter, also referred to as the "operation period"), the dross defect can be effectively suppressed.

First, the present inventors focused attention on the growth rate of each bottom ash in order to suppress the formation of middle bottom ash. The bottom dross and Γ of (B) to (D) above ₂ In the phase zinc slag, gamma is ₂ Quickest phase zinc dross, delta ₁ The phase zinc dross is the slowest. Thus, Γ ₂ Phase zinc slag ratio delta ₁ The growth of the phase zinc slag is fast and far earlier than delta ₁ Phase of zinc dross, gamma ₂ The grain size of the phase zinc slag exceeds 100 mu m. In contrast, δ ₁ The growth speed of the phase zinc slag is obviously slower than that of gamma ₂ Growth rate of the zinc dross. Therefore, even delta ₁ Nucleation of the phase zinciferous slag, delta ₁ The phase zinc dross is difficult to be like gamma ₂ Grow as fast as it does. Therefore, it is considered that during the hot dip galvanizing process (during operation), the gap is equal to Γ ₂ The phase zinciferous slag is preferably formed in the range of delta ₁ The phase generation region is subjected to hot dip galvanizing treatment.

Accordingly, the present inventors have found that the bath temperature T (. Degree. C) of the hot dip galvanizing bath and the free Al concentration C of the hot dip galvanizing bath are controlled _A1 (mass%) and the state of the formed zinc dross were further investigated and investigated. As a result, the present inventors have prepared a metastable state diagram of the zinc dross in the hot dip galvanizing bath shown in fig. 1. Fig. 1 will be described below.

The vertical axis of FIG. 1 represents the free Al concentration C in the hot dip galvanizing bath _A1 (mass%). Here, in the present specification, the "concentration C of free Al in the hot dip galvanizing bath _Al "means the concentration (mass%) of free Al melted in the hot dip galvanizing bath. That is, in the present specification, the "concentration of free Al in hot dip galvanizing bath C _Al "means that the slag is contained in the slag other than the zinc slag (top slag and bottom slag)In addition to the Al content, the concentration (mass%) of free Al melted in the hot dip galvanizing bath (i.e., in the liquid phase). The horizontal axis in FIG. 1 represents the bath temperature T (. Degree. C.) in the hot dip galvanizing bath.

Referring to FIG. 1, the free Al concentration C shown in FIG. 1 _Al In the range and bath temperature T (. Degree. C.), there are regions 1A where dross is formed in the hot dip galvanizing bath (hereinafter referred to as dross forming regions 1A) and Γ ₂ Region 2 where phase zinciferous slag is formed (hereinafter referred to as "gamma") ₂ Phase generation regions 2) and δ ₁ Region 3 where phase zinciferous slag is formed (hereinafter referred to as "δ ₁ Phase generation region 3).

Top dross forming regions 1A and Γ ₂ The phase generation region 2 passes through a phase change line F ₁₂ And (4) dividing. Top slag forming regions 1A and δ ₁ The phase generation region 3 passes through a phase change line F ₁₃ And (5) dividing. Gamma-shaped ₂ Phase generation regions 2 and δ ₁ The phase generation region 3 passes through a phase change line F ₂₃ And (5) dividing.

For example, at a bath temperature T of 440 ℃, the free Al concentration C _A1 Gamma in 0.135% hot dip galvanizing bath ₂ And (4) generating phase zinc slag. Assuming that the concentration C of free Al in the hot dip galvanizing bath _A1 The bath temperature T was increased from 440 ℃ to 470 ℃ while maintaining the temperature at 0.135%. In this case, the state of the hot dip galvanizing bath is from Γ ₂ The phase generation region 2 crosses the phase change line F ₂₃ To be transferred to delta ₁ A phase generation region 3. Thus, Γ in a hot dip galvanizing bath ₂ The phase zinc slag is transformed into delta ₁ And (4) phase zinc slag. Further, it is assumed that the bath temperature T is 440 ℃ and the free Al concentration C _A1 Free Al concentration C of hot-dip galvanizing bath of 0.135% _A1 Increase to 0.140%. In this case, the state of the hot dip galvanizing bath is from Γ ₂ The phase generation region 2 crosses the phase change line F ₁₂ And transferred to the top dross forming area 1A. Thus, Γ in a hot dip galvanizing bath ₂ The phase zinc slag is transformed into top slag.

The present inventors have also found that Γ of the metastable state diagram shown in FIG. 1 ₂ Gamma is present in the phase generation region 2 ₂ Nucleus generation region 21 and Γ ₂ Boundary line F demarcated by grain growth region 22 ₂₁₂₂ . The present inventors have also found thatShown as delta of the metastable state diagram ₁ The presence of delta in the phase generation region 3 ₁ Nuclear generation regions 31 and δ ₁ Boundary line F demarcated by grain growth region 32 ₃₁₃₂ . This point will be explained below.

At Γ type ₂ In the phase generation region 2, Γ ₂ The kernel generation region 21 corresponds to Γ ₂ The grain growth region 22 is located at the boundary line F ₂₁₂₂ The low temperature side of (1). At Γ type ₂ In the kernel generation region 21, and ₂ grain growth region 22 promotes gamma in hot dip galvanizing bath ₂ And generating nuclei of the phase zinc slag. I.e. promoting microfine Γ ₂ And (3) generation of the zinc dross. On the other hand, in ₂ In the grain growth region 22, with F ₂ The nucleation region 21 promotes the gamma rays already present in the hot-dip galvanizing bath, as compared with the nucleation region 21 ₂ Phase growth (grain growth).

Likewise, at δ ₁ In the phase generation region 3, with respect to δ ₁ Grain growth region 32, delta ₁ The kernel generation area 31 is located on the boundary line F ₃₁₃₂ The high temperature side of (a). At delta ₁ In the nucleus generation region 31, and δ ₁ The grain growth zone 32 promotes delta in the hot dip galvanizing bath ₁ Nuclei of the phase zinc dross are generated. Namely, the fine δ is promoted ₁ And (4) generation of phase zinc slag. On the other hand, at δ ₁ In the grain growth region 32, and ₁ the nucleation region 31 promotes the delta existing in the hot dip galvanizing bath ₁ Growth of the phase (grain growth).

The phase change line F in the meta-stable state diagram can be defined by the following formula (A) ₂₃ 。

F ₂₃ ＝577.24×C _Al +382.49 (A)

Further, the boundary F can be defined by the following formula (B) ₂₁₂₂ 。

F ₂₁₂₂ ＝466.15×C _Al +385.14 (B)

Further, the boundary line F can be defined by the following formula (C) ₃₁₃₂ 。

F ₃₁₃₂ ＝390.91×C _Al +414.20 (C)

Here, among the formulae (A) to (C)Is "C" of _Al "free Al concentration C in hot dip galvanizing bath _Al (mass%).

The present inventors have studied a hot dip galvanizing treatment method capable of suppressing the dross defect based on the metastable state diagram of fig. 1. As described above, the zinciferous slag defects are caused by the middle-sized bottom dross having a particle size of 100 μm or more and less than 300. Mu.m. During the hot dip galvanizing treatment period (during the operation period), the free Al concentration C in the hot dip galvanizing bath is adjusted to suppress the formation of middle-sized bottom dross _Al And a bath temperature T so that the state of the hot dip galvanizing bath becomes δ in FIG. 1 ₁ The kernel generation area 31. At delta ₁ In the nucleus generation region 31, though δ ₁ Nucleation of the zinciferous slag is promoted, however, delta ₁ The growth of the phase zinciferous slag is inhibited. Further, as described above, in the bottom slag, δ ₁ The growth rate of the phase zinc slag is the slowest. Therefore, if the hot dip galvanizing bath during operation is set to δ ₁ The nucleus generation region 31 can suppress bottom dross (in this case, δ) ₁ Phase zinc dross) grows as medium-sized bottom dross.

However, even if the hot dip galvanizing bath is kept at δ during operation ₁ In the nucleation region 31, if the period of execution (operation period) of the hot-dip galvanizing process is long, the fine δ ₁ The phase dross also grows to some extent. Therefore, if the hot dip galvanizing bath during the operation is simply set to δ ₁ In the nucleus production zone 31, if the operation period is long, medium-sized bottom ash may be produced.

When the amount of the middle-sized bottom dross increases in the hot dip galvanizing bath, if an operation of removing the bottom dross from the molten zinc pot (hereinafter referred to as a bottom dross removing step) is performed, the formation of a dross defect can be suppressed. However, when the bottom dross removing step is performed, it is necessary to stop the hot dip galvanizing process and stop the continuous hot dip galvanizing line facility. In this specification, such a state in which the hot dip galvanizing process is stopped is referred to as "shutdown". When the bottom slag removing step is employed, the production efficiency is lowered if the frequency of the bottom slag removing step is increased.

Therefore, the present inventors have found that when the hot dip galvanizing treatment is performed in the bottom dross removing operation, the frequency of the bottom dross removing step can be suppressed, and the hot dip galvanizing treatment can be performed with high accuracyFurther studies were made on a method capable of sufficiently suppressing the defects of zinc dross. As a result, the present inventors have found that: by intentionally and preliminarily containing coarse bottom dross with a particle size of 300 [ mu ] m or more in a hot dip galvanizing bath, the resultant is treated with a flux of delta ₁ The formation of the middle-size bottom dross can be suppressed for a long time by performing the hot-dip galvanizing treatment in the nucleation area 31. This point will be explained below.

When the bottom dross reaches a certain size, the bottom dross grows by ostwald growth. The ostwald growth is a phenomenon in which when metal particles of the same kind having different particle diameters are present in a matrix phase (Zn in a liquid phase in the present specification), the metal particles having small particle diameters shrink or disappear, and the metal particles having large particle diameters grow further coarsely.

In the present embodiment, coarse bottom dross having a particle size of 300 μm or more is contained in advance in the hot dip galvanizing bath before the hot dip galvanizing treatment is performed. Then, a hot dip galvanizing bath containing coarse bottom dross is used at delta ₁ The nucleation region 31 is subjected to hot dip galvanizing treatment. In this case, during the execution of the hot dip galvanizing process, though δ ₁ Nucleation of the zinc dross but formation of a nucleation delta ₁ The phase zinc dross shrinks or disappears and is coarse (in this case, coarse delta) ₁ Phase zinc dross). That is, when a hot dip galvanizing bath containing coarse bottom dross in advance is used, the fine δ ₁ The phase zinc dross is coarsened by coarse bottom dross (coarsened. Delta.) by Ostwald growth ₁ Phase zinc dross). Even if the coarse bottom dross grows further, the coarse bottom dross is not easily curled up by the accompanying flow because of its large mass. In the case of this method, even if the hot-dip galvanizing treatment is performed for a long period of time, the fine δ can be suppressed ₁ And (3) generation of the zinc dross. Therefore, the formation of the medium zinc dross is further suppressed. As a result, the occurrence of the zinciferous residue defects can be suppressed even if the operation period is long.

In short, in the present embodiment, after coarse bottom dross is included in the hot dip galvanizing bath in advance, δ is set ₁ The nucleation region 31 is subjected to hot dip galvanizing treatment. In this case, the coarse bottom dross grows further due to Ostwald growth, and thus not only the fine bottom dross (fine bottom dross) can be suppressedδ ₁ Phase zinciferous slag) and also can inhibit its formation. As a result, the middle-sized bottom dross, which is a cause of the dross defect, can be effectively suppressed.

As described above, the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment is completed based on a technical idea completely different from the conventional art as follows: in this state, the hot dip galvanizing treatment is performed with coarse bottom dross, which has been conventionally considered to be an object to be removed, intentionally included in the hot dip galvanizing treatment bath. Specifically, the method for producing the hot-dip galvanized steel sheet according to the present embodiment is as follows.

[1] The method for producing a hot-dip galvanized steel sheet according to (1) comprises:

a coarse bottom dross generation step of adjusting the free Al concentration C in the hot dip galvanizing bath so as to satisfy the formula (1) _Al And a bath temperature T, thereby generating coarse bottom dross with a particle size of 300 [ mu ] m or more in the hot dip galvanizing bath; and

466.15×C _Al +385.14≤T≤577.24×C _Al +382.49 (1)

390.91×C _Al +414.20≤T≤485.00 (2)

[2] The method for producing a hot-dip galvanized steel sheet according to [1], wherein,

and performing the coarse bottom dross producing step on the hot-dip galvanizing bath after the hot-dip galvanizing treatment step, while a machine is stopped when the hot-dip galvanizing treatment step is stopped.

[3] The method for producing a hot-dip galvanized steel sheet according to item [1] or [2], wherein,

in the hot dip galvanizing treatment step, the bath temperature T of the hot dip galvanizing bath after the coarse bottom dross generation step is increased, thereby producing the hot dip galvanizing bath satisfying formula (2).

[4] The method for producing a hot-dip galvanized steel sheet according to [3], further comprising repeating the coarse bottom dross forming step and the hot-dip galvanizing treatment step alternately,

in the case where the coarse bottom dross producing step is performed after the hot-dip galvanizing treatment step, the hot-dip galvanizing bath satisfying formula (1) is prepared by decreasing the bath temperature T of the hot-dip galvanizing bath after the hot-dip galvanizing treatment step in the coarse bottom dross producing step.

[5] The method for producing a hot-dip galvanized steel sheet according to any one of [1] to [4], wherein

The concentration C of the free Al in the hot dip galvanizing bath in the coarse bottom dross forming step and the hot dip galvanizing treatment step _Al Is set to 0.125 mass% or more.

[6]The method for producing a hot-dip galvanized steel sheet according to [5]]The method for producing a hot-dip galvanized steel sheet, wherein the concentration C of free Al in the hot-dip galvanizing bath in the coarse bottom dross forming step and the hot-dip galvanizing treatment step is set to be lower than that in the hot-dip galvanizing bath in the hot-dip galvanizing treatment step _Al The content is 0.138 mass% or less.

[7] The method for producing a hot-dip galvanized steel sheet according to any one of [1] to [6], further comprising a bottom dross removal step of removing at least a part of the coarse bottom dross in the hot-dip galvanizing bath before the coarse bottom dross generation step is performed.

[8] The method for producing an alloyed hot-dip galvanized steel sheet according to (1) comprises:

a step of manufacturing the hot-dip galvanized steel sheet by performing the method for manufacturing a hot-dip galvanized steel sheet according to any one of [1] to [7 ]; and

and an alloying treatment step of alloying the hot-dip galvanized steel sheet.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[ Structure of Hot-Dip galvanizing line Equipment ]

Fig. 2 is a functional block diagram showing an example of the overall configuration of a hot-dip galvanizing line facility for manufacturing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet. Referring to fig. 2, the hot-dip galvanizing line facility 1 includes: an annealing furnace 20, a hot-dip galvanizing facility 10, and a temper rolling mill (finisher) 35.

The annealing furnace 20 includes: not shown in the figure, 1 or more heating zones, and 1 or more cooling zones disposed downstream of the heating zones. In the annealing furnace 20, the steel sheet is supplied to the heating zone of the annealing furnace 20, and the steel sheet is annealed. The annealed steel sheet is cooled in a cooling zone and conveyed to a hot dip galvanizing facility 10. The hot-dip galvanizing facility 10 is disposed downstream of the annealing furnace 20. In the hot-dip galvanizing facility 10, a steel sheet is subjected to a hot-dip galvanizing treatment to produce an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet. The temper rolling mill 35 is arranged downstream of the hot dip galvanizing plant 10. In the temper rolling mill 35, the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet manufactured in the hot-dip galvanizing facility 10 is lightly rolled down as necessary to adjust the surface of the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet.

[ with regard to the hot dip galvanizing facility 10]

Fig. 3 is a side view of the hot-dip galvanizing apparatus 10 in fig. 1. Referring to fig. 3, the hot-dip galvanizing facility 10 includes, for example: a molten zinc pot 101, an immersion roller 107, a support roller 113, a gas wiping device 109, and an alloying furnace 111.

The inside of the annealing furnace 20 disposed in the front stage of the hot dip galvanizing facility 10 is constantly maintained in a reducing atmosphere. In the annealing furnace 20, the steel sheet S continuously conveyed is heated. The steel sheet S is heated in the annealing furnace 20, whereby the surface of the steel sheet S is activated and the mechanical properties of the steel sheet S are adjusted. The downstream end portion of the annealing furnace 20 corresponding to the side from which the annealing furnace 20 is separated has a space in which the draw-down rolls 30 are disposed. The downstream end of the annealing furnace 20 is connected to the upstream end of the long nozzle 105. The downstream end portion of the long nozzle 105 is immersed in the hot dip galvanizing bath 103. The interior of the long nozzle 105 is blocked from the atmospheric atmosphere, and a reducing atmosphere is maintained.

The steel sheet S, which has been conveyed downward by the lower rotary rolls 30, passes through the long nozzle 105 and is continuously immersed in the hot dip galvanizing bath 103 stored in the molten zinc pot 101. The submerged roller 107 is disposed inside the molten zinc pot 101. The submerged roller 107 has a rotation axis parallel to the width direction of the steel sheet S. The width of the submerged roller 107 in the axial direction is larger than the width of the steel sheet S. The submerged roller 107 contacts the steel sheet S and shifts the traveling direction of the steel sheet S to the upper side of the hot dip galvanizing facility 10.

The support roller 113 is a well-known member. The support roll 113 is disposed in the hot dip galvanizing bath 103 above the immersion roll 107. The support roller 113 includes a pair of rollers. The pair of support rollers 113 has a rotation axis parallel to the width direction of the steel sheet S. The support rollers 113 support the steel sheet S conveyed upward while sandwiching the steel sheet S whose traveling direction is shifted upward by the submerged rollers 107.

The gas wiping device 109 is disposed above the immersion roller 107 and the support roller 113 and above the liquid surface of the hot dip galvanizing bath 103. The gas wiping device 109 includes a pair of gas ejecting devices. The pair of gas injection devices have gas injection nozzles facing each other. During the hot dip galvanizing process, the steel sheet S passes between the pair of gas injection nozzles of the gas wiping apparatus 109. At this time, the pair of gas injection nozzles face the surface of the steel sheet S. The gas wiping device 109 blows gas to both surfaces of the steel sheet S drawn up from the hot-dip galvanizing bath 103. Thereby, the gas wiping device 109 scrapes off a part of the hot-dip galvanized steel sheets adhering to both surfaces of the steel sheet S, and adjusts the amount of the hot-dip galvanized steel sheets adhering to the surface of the steel sheet S.

The alloying furnace 111 is disposed above the gas wiping device 109. The alloying furnace 111 passes the steel sheet S, which has passed through the gas wiping device 109 and is conveyed upward, through the inside thereof, and performs an alloying treatment on the steel sheet S. The alloying furnace 111 includes a heating zone, a soaking zone, and a cooling zone in this order from the entry side toward the exit side of the steel sheet S. The heating zone heats the steel sheet S so that the temperature (sheet temperature) thereof is substantially uniform. The heat-retaining section retains the plate temperature of the steel plate S. At this time, the hot-dip galvanized layer formed on the surface of the steel sheet S is alloyed to become an alloyed hot-dip galvanized layer. The cooling zone cools the steel sheet S formed with the alloyed hot-dip galvanized layer. As described above, the alloying furnace 111 performs the alloying treatment by using the heating zone, the soaking zone, and the cooling zone. In the case of manufacturing an alloyed hot-dip galvanized steel sheet, the alloying furnace 111 performs the alloying treatment described above. On the other hand, in the case of manufacturing a hot-dip galvanized steel sheet, the alloying furnace 111 does not perform alloying treatment. In this case, the steel sheet S passes through the alloying furnace 111 which is not in operation. Here, the non-operation means, for example, a state in which the alloying furnace 111 is kept in an on-line arrangement and the power supply is stopped (non-activated state). The steel sheet S passing through the alloying furnace 111 is conveyed to the next step by the upper roller 40.

When manufacturing a hot-dip galvanized steel sheet, as shown in fig. 4, the alloying furnace 111 may be moved offline. In this case, the steel sheet S is conveyed from the upper roll 40 to the next step without passing through the alloying furnace 111.

When the hot-dip galvanizing facility 10 is a facility dedicated to hot-dip galvanized steel sheets, the hot-dip galvanizing facility 10 may not include the alloying furnace 111 as shown in fig. 5.

[ other configuration examples of the facilities of the hot-dip galvanizing line ]

The hot-dip galvanizing line facility 1 is not limited to the configuration shown in fig. 2. For example, when a Ni layer is formed on a steel sheet by performing a Ni pre-plating treatment on the steel sheet before a hot-dip galvanizing treatment, a Ni pre-plating facility 45 may be disposed between the annealing furnace 20 and the hot-dip galvanizing facility 10, as shown in fig. 6. The pre-Ni plating apparatus 45 includes a Ni plating tank for storing a Ni plating bath. The Ni pre-plating treatment is performed by an electroplating method. The hot-dip galvanizing line facility 1 shown in fig. 2 and 6 includes an annealing furnace 20 and a temper rolling mill 35. However, the hot-dip galvanizing line facility 1 may not include the annealing furnace 20. The hot-dip galvanizing line facility 1 may not include the temper mill 35. The hot-dip galvanizing line facility 1 may include at least a hot-dip galvanizing facility 10. The annealing furnace 20 and the temper rolling mill 35 may be disposed as necessary. The hot-dip galvanizing line facility 1 may include a pickling facility for pickling a steel sheet upstream of the hot-dip galvanizing facility 10, or may include facilities other than the annealing furnace 20 and the pickling facility. The hot-dip galvanizing line facility 1 may be provided with facilities other than the temper rolling mill 35 at a position downstream of the hot-dip galvanizing facility 10.

[ method for producing Hot-Dip galvanized Steel sheet according to the embodiment ]

[ concerning the hot-dip galvanizing line facilities utilized ]

In the hot-dip galvanizing treatment method of the present embodiment, the hot-dip galvanizing line facility 1 is used. The hot-dip galvanizing line facility 1 has a structure shown in fig. 2 and 6, for example. As described above, the hot-dip galvanizing line facility 1 used in the hot-dip galvanizing treatment method according to the present embodiment may be the facility shown in fig. 2 and 6, or the facility shown in fig. 2 and 6 may be a facility further supplemented with another configuration. As described above, the hot-dip galvanizing line facility 1 may not include the annealing furnace 20. The hot-dip galvanizing line facility 1 may not include the temper mill 35. The hot-dip galvanizing line facility 1 may include at least a hot-dip galvanizing facility 10. A known hot-dip galvanizing line facility 1 having a different structure from that of fig. 2 and 6 may be used.

[ Steel sheet to be subjected to Hot-Dip galvanizing treatment ]

The steel type and size (plate thickness, plate width, etc.) of the steel sheet (base steel sheet) used in the hot dip galvanizing method of the present embodiment are not particularly limited. The steel sheet may be a known steel sheet suitable for an alloyed hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet, depending on various mechanical properties (for example, tensile strength, workability, and the like) required for the alloyed hot-dip galvanized steel sheet or the hot-dip galvanized steel sheet to be produced. A steel sheet used for an automobile outer panel may be used as a steel sheet to be subjected to hot dip galvanizing. The steel sheet (base steel sheet) to be subjected to the hot-dip galvanizing treatment in the present embodiment may be a hot-rolled steel sheet or a cold-rolled steel sheet.

[ Hot-dip galvanizing bath ]

The main component of the hot dip galvanizing bath is Zn. The hot dip galvanizing bath contains Al in addition to Zn. That is, the hot dip galvanizing bath used in the hot dip galvanizing treatment method of the present embodiment is a plating solution containing Al at a specific concentration and the balance of Zn and impurities. The impurity is, for example, fe. When the hot dip galvanizing bath contains Al at a specific concentration, excessive reaction of Fe and Zn in the bath can be suppressed. As a result, it is possible to suppress uneven alloy reaction between the steel sheet immersed in the hot dip galvanizing bath and Zn.

As shown in FIG. 1, the free Al concentration C _Al A preferred lower limit of (c) is 0.125%. If the free Al concentration is C _Al When the content is 0.125 mass% or more, excessive alloying of the hot-dip galvanized layer can be suppressed in the alloying treatment. Therefore, embrittlement of the alloyed hot-dip galvanized layer due to excessive alloying can be suppressed. As a result, the adhesiveness of the alloyed hot-dip galvanized layer to the steel sheet is improved. Concentration of free Al C _Al More preferable lower limit of (b) is 0.127%, more preferably 0.129%, and still more preferably 0.130%.

Concentration of free Al C _Al The preferable upper limit of (b) is 0.138% by mass or less. In this case, alloying can be performed more efficiently, and an alloyed hot-dip galvanized layer can be sufficiently formed. Concentration of free Al C _Al The more preferable upper limit of (b) is 0.137%, more preferably 0.136%, and still more preferably 0.135%.

The concentration of free Fe in hot dip galvanizing bath 103 is not particularly limited. The free Fe concentration is, for example, 0.020 to 0.060% by mass%. Fe in hot dip galvanizing bath 103 may be eluted from steel sheet S, or may be contained in hot dip galvanizing bath 103 for other reasons. The hot dip galvanizing bath 103 may contain impurities other than Fe. The impurities referred to herein are components that are mixed due to other causes such as raw materials, and are allowed within a range that does not adversely affect the production method of the present embodiment.

[ method of measuring the concentration of free Al and the concentration of free Fe in the hot-dip galvanizing bath 103 ]

The method for determining the free Al concentration and the free Fe concentration in the hot dip galvanizing bath 103 is not particularly limited. For example, the free Al concentration C is obtained from the Al concentration and Fe concentration obtained by Inductively Coupled Plasma (ICP: inductively Coupled Plasma) emission spectrometry _Al (% by mass) and free Fe concentration (% by mass).

Specifically, a sample is collected from the hot dip galvanizing bath 103. Will be provided withThe sample cooled rapidly and solidified. Using the solidified sample, al concentration and Fe concentration were obtained by ICP emission spectrometry. The Al concentration obtained by ICP emission spectrometry contains not only the free Al concentration in the hot dip galvanizing bath but also the Al concentration in the zinc dross. That is, the Al concentration obtained by ICP emission spectrometry is the so-called total Al concentration. Similarly, the Fe concentration obtained by the above ICP emission spectrometry includes not only the free Fe concentration in the hot dip galvanizing bath but also the Fe concentration in the zinc dross. That is, the Fe concentration obtained by ICP emission spectrometry is a so-called total Fe concentration. Then, the obtained total Al concentration and total Fe concentration are used to determine the free Al concentration C by using the well-known Zn-Fe-Al ternary system state diagram _Al And free Fe concentration.

Concentration of free Al C _Al And the free Fe concentration was determined as follows. Preparing a Zn-Fe-Al ternary system state diagram at the bath temperature T when a sample is collected. As described above, the Zn — Fe — Al ternary system state diagram is well known and is also disclosed in fig. 2 and 3 of non-patent document 1. Non-patent document 1 is a well-known article among researchers and developers of hot-dip galvanizing baths. On the Zn-Fe-Al ternary system state diagram, points determined from the total Al concentration and the total Fe concentration obtained by the ICP emission spectrometry method were marked. Then, a connecting line (conjugate line) is drawn from the marked point toward the liquidus line in the Zn-Fe-Al ternary system state diagram. The Al concentration at the intersection of the liquidus line and the connecting line is defined as the free Al concentration C _Al The Fe concentration at the intersection of the liquidus line and the connecting line is defined as the free Fe concentration.

By the above method, the free Al concentration C in the hot dip galvanizing bath can be determined _Al And the free Fe concentration in the hot dip galvanizing bath. In the chemical composition of the hot dip galvanizing bath, the free Al concentration C _Al And the balance other than the free Fe concentration may be regarded as Zn.

The method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment includes: a coarse bottom slag generating step (S1) and a hot-dip galvanizing treatment step (S2). Hereinafter, each step will be explained.

[ procedure (S1) for producing coarse bottom dross ]

The coarse bottom dross forming step (S1) is performed while the hot dip galvanizing treatment is not performed. That is, for example, the coarse bottom dross producing step (S1) is performed while the hot-dip galvanizing line facility is stopped (stopped period) without the steel sheet passing through the hot-dip galvanizing facility.

In the coarse bottom dross forming step (S1), the free Al concentration C in the hot dip galvanizing bath is adjusted so as to satisfy the formula (1) _Al And a bath temperature T, thereby generating coarse bottom dross with a particle size of 300 [ mu ] m or more in the hot dip galvanizing bath.

466.15×C _Al +385.14≤T≤577.24×C _Al +382.49 (1)

Here, ` C ` in the formula (1) _Al "free Al concentration C in Hot Dip galvanizing bath 103 _Al (mass%).

"466.15 XC in formula (1) _Al +385.14 "corresponds to formula (B) above. I.e., 466.15 XC _Al +385.14 corresponds to the boundary line F in FIG. 1 ₂₁₂₂ . "577.24 XC in formula (1) _Al +382.49 "corresponds to formula (a). I.e., 577.24 XC _Al +382.49 corresponds to phase change line F in FIG. 1 ₂₃ . Accordingly, equation (1) represents Γ in fig. 1 ₂ A grain growth region 22.

In the coarse bottom dross forming step (S1), the concentration C of free Al in the hot dip galvanizing bath is adjusted _Al And a bath temperature T for maintaining the state of hot dip galvanizing bath 103 at Γ ₂ A grain growth region 22. At this time, the bottom dross formed in the hot dip galvanizing bath 103 is Γ ₂ And (5) phase zinc slag. As mentioned above, Γ ₂ The growth speed of the phase zinc slag in the bottom slag is the fastest. Further, F ₂ Grain growth region 22 and Γ ₂ The nucleus generation region 21 promotes Γ compared to ₂ And (4) growing the phase zinc slag. Therefore, in the coarse bottom slag producing step (S1), coarse bottom slag having a particle size of 300 μm or more can be produced in a short period of time.

In the coarse bottom dross growth step, the period during which the hot dip galvanizing bath 103 is maintained so as to satisfy formula (1) is not particularly limited. So long as it can form coarse particles having a particle size of 300 μm or moreAnd (4) bottom slag. It was confirmed that formation of Γ (mm) having a particle size of 300 μm or more was achieved by maintaining hot dip galvanizing bath 103, which is a newly built bath without bottom dross, for at least 30 days under the condition that expression (1) is satisfied ₂ And (4) phase zinc slag. Therefore, in the coarse bottom dross growing step, it is preferable to maintain hot dip galvanizing bath 103 for at least 30 days so as to satisfy formula (1). More preferably, in the coarse bottom dross growth step, the hot dip galvanizing bath 103 is maintained for at least 60 days, and still more preferably for at least 90 days so as to satisfy formula (1).

In the present specification, the particle size of the bottom slag is defined as follows. Referring to fig. 7, in each of the bottom dross 100, the largest line segment LS among line segments LS connecting arbitrary 2 points of the interface 150 between the bottom dross 100 and the matrix 200 (i.e., the outer periphery of the zinc dross) is defined as "particle size". The particle size can be determined by image processing on a photographic image of the observation field. In the present specification, the dross having a longest diameter of less than 20 μm is excluded because it hardly affects the dross defect.

[ Hot-Dip galvanizing treatment Process (S2) ]

In the hot dip galvanizing process (S2), the steel sheet is subjected to a hot dip galvanizing process using the hot dip galvanizing bath 103 after the coarse bottom dross forming process (S1). Specifically, the steel sheet is passed through a hot dip galvanizing bath 103 containing coarse bottom dross. At this time, a hot-dip galvanized layer is formed on the surface of the steel sheet.

In the hot dip galvanizing treatment step (S2), the free Al concentration C in the hot dip galvanizing bath is adjusted so as to satisfy the formula (2) during the period in which the molten zinc treatment is performed (i.e., during the operation period) _Al And bath temperature T.

390.91×C _Al +414.20≤T≤485.00 (2)

Here, ` C ` in the formula (2) _Al "position substituted into free Al concentration C in hot dip galvanizing bath 103 _Al (mass%).

"390.91 XC" in formula (2) _Al +414.20 "corresponds to formula (C) above. I.e., 390.91 XC _Al +414.20 corresponds to the boundary line F in FIG. 1 ₃₁₃₂ 。

Briefly, hot dip galvanisingIn treatment step (S2), bath temperature T of hot dip galvanizing bath 103 after coarse bottom dross formation step (S1) is increased to change the state of hot dip galvanizing bath 103 from Γ ₂ The grain growth region 22 moves to δ ₁ A phase kernel generation region 31. Then, the state of hot dip galvanizing bath 103 is maintained at δ ₁ A phase kernel generation region 31. At this time, coarse bottom dross in hot dip galvanizing bath 103 is t from ₂ Phase change to delta ₁ And (4) phase(s). During phase transition, part or all of the coarse bottom slag is not dissolved. That is, when the process shifts from the coarse bottom dross growth step (S1) to the hot dip galvanizing treatment step (S2), the coarse bottom dross deposited on the bottom of the molten zinc pot 101 flows from F ₂ Phase change to delta ₁ Without significantly changing the particle size and shape.

As described above, in the hot dip galvanizing treatment step (S2), the bath temperature T of hot dip galvanizing bath 103 is increased to change the state of hot dip galvanizing bath 103 from Γ ₂ The grain growth region 22 moves to δ ₁ The kernel generation area 31. Then, the state of the hot dip galvanizing bath 103 is maintained at δ ₁ The kernel generation area 31. At this time, the bottom of the molten zinc pot 101 is provided with a gap from gamma ₂ Phase change to delta ₁ Coarse bottom slag of phase. That is, in the hot dip galvanizing treatment step (S2), hot dip galvanizing treatment is performed using a hot dip galvanizing bath 103 containing coarse bottom dross.

In the hot-dip galvanizing process (S2), the state of the hot-dip galvanizing bath 103 is δ during the period of performing the hot-dip galvanizing treatment (i.e., during the operation period) ₁ The kernel generation area 31. Therefore, in the hot dip galvanizing treatment step, fine δ is generated in the hot dip galvanizing bath 103 ₁ And (5) phase zinc slag. However, coarse bottom dross is present at the bottom of the molten zinc pot 101. Thus, by Ostwald growth, the delta is fine ₁ The phase zinc slag shrinks or disappears, and coarse bottom slag grows. That is, in the hot dip galvanizing treatment step (S2), the fine bottom dross (fine δ) is suppressed by the ostwald growth of the coarse bottom dross ₁ Phase zinc dross) formation and growth. In this case, the fine δ ₁ The phase zinc slag becomes smaller and the coarse bottom slag becomes larger. As a result, the middle-sized bottom sediment (. Delta.) having a particle size of 100 to 300 μm can be suppressed ₁ Phase zinc dross). In addition, hot dip galvanizingDuring the execution of the treatment step (S2), coarse bottom dross further grows. However, since the coarse bottom dross has a large mass, it is not easily rolled up in the hot dip galvanizing bath 103 with the accompanying flow. Therefore, the possibility of the coarse bottom dross adhering to the steel sheet is extremely low.

As can be seen, in the hot dip galvanizing treatment step (S2), even if fine bottom dross (δ) is generated in the hot dip galvanizing treatment ₁ Phase zinc dross) is formed by using the Ostwald growth of coarse bottom dross, thereby effectively inhibiting the growth of fine bottom dross into medium-sized bottom dross. Therefore, formation of a dross defect in a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet can be suppressed.

In the hot dip galvanizing process (S2), even if the hot dip galvanizing process is performed for a long time, the formation of the middle-sized bottom dross can be effectively suppressed. Therefore, the bottom ash removal step need not be performed, and the frequency of performing the bottom ash removal step can be suppressed. That is, the frequency of stopping the hot dip galvanizing facility 10 (shutdown frequency) can be suppressed. Therefore, the production efficiency can be improved.

[ timing for carrying out the coarse bottom dross producing step (S1) ]

In the present embodiment, when newly constructing the hot dip galvanizing bath 103, a coarse bottom dross forming step (S1) is performed before the hot dip galvanizing treatment step (S2).

On the other hand, when the hot dip galvanizing treatment step (S2) is performed for a long time, coarse bottom dross deposited on the bottom of the molten zinc pot 101 excessively grows, and the amount of deposition of the coarse bottom dross becomes excessive. At this time, the molten zinc pot 101 may be taken out of the hot dip galvanizing facility 10 and subjected to a bottom dross removing step. In the bottom dross removal step, at least a part or all of coarse bottom dross in hot dip galvanizing bath 103 is removed. The method for removing the bottom slag may be performed by a known method. In the bottom ash removal step, for example, a bucket suspended from a crane is immersed in the molten zinc pot 101, and the bottom ash is scooped up by the bucket. Thereafter, the bucket is pulled up from the molten zinc pot 101, and the bottom slag scooped up by the bucket is taken out to the outside of the molten zinc pot 101. Alternatively, the bottom slag may be removed by pouring Al into the molten zinc pot 101 to form a top slag from the bottom slag.

The hot dip galvanizing bath 103 from which the bottom dross deposited at the bottom of the molten zinc pot 101 has been removed is subjected to a coarse bottom dross formation step (S1). As a result, coarse bottom dross is again produced in the hot dip galvanizing bath 103 after the bath adjustment. Then, the hot-dip galvanizing bath 103 containing the coarse bottom dross and having been adjusted is set in the hot-dip galvanizing facility 10 again. Then, a hot dip galvanizing treatment step (S2) is performed. In short, the coarse bottom dross forming step (S1) and the hot dip galvanizing step (S2) are repeatedly performed a plurality of times.

[ free Al concentration C of Hot Dip galvanizing bath 103 _Al Adjustment method of bath temperature T]

The free Al concentration C of hot dip galvanizing bath 103 in the coarse bottom dross forming step (S1) and the hot dip galvanizing treatment step (S2) _Al And the bath temperature T may be adjusted by a known method.

For example, the free Al concentration C in the hot dip galvanizing bath is adjusted by adding Al to the hot dip galvanizing bath _Al . The Al addition is performed, for example, by immersing an Al ingot in a hot-dip galvanizing bath. The Al addition may be performed by a method other than dipping the Al ingot in a hot dip galvanizing bath. When Al is added to a hot dip galvanizing bath by immersing an Al ingot in the hot dip galvanizing bath, the Al ingot is immersed in the hot dip galvanizing bath at an immersion speed that can suppress rapid changes in the temperature of the hot dip galvanizing bath. Concentration C of free Al in hot-dip galvanizing bath _Al The adjustment method of (2) is not limited to the above-described method. Concentration C of free Al in hot-dip galvanizing bath _Al The adjustment method (2) may be a known method.

The bath temperature T of hot dip galvanizing bath 103 is adjusted by using a heating device provided in hot dip galvanizing pot 101. The heating device is, for example, a high-frequency induction heating device.

When the process shifts from the coarse bottom dross producing step (S1) to the hot dip galvanizing treatment step (S2), the bath temperature of the hot dip galvanizing bath 103 can be changed from the hot dip galvanizing bath 103 satisfying the equation (1) to the hot dip galvanizing bath 103 satisfying the equation (2) easily. Specifically, referring to fig. 1, in the coarse bottom dross forming step (S1), the state of the hot dip galvanizing bath 103 (free Al concentration C) _A1 And bath temperature T) is at gamma ₂ Within the confines of the grain growth region 22. Here, if the bath temperature T is increased at the time of transition from the coarse bottom dross forming region (S1) to the hot dip galvanizing treatment step (S2), the state of the hot dip galvanizing bath 103 can be easily changed from Γ ₂ Transformation of grain growth region into delta ₁ The kernel generates a region. That is, the hot-dip galvanizing bath 103 satisfying the formula (1) can be easily changed to the hot-dip galvanizing bath 103 satisfying the formula (2) simply by changing the bath temperature T.

In the present embodiment, the coarse bottom dross forming step (S1) and the hot dip galvanizing step (S2) may be alternately repeated. When the coarse bottom dross producing step (S1) is performed after the hot dip galvanizing treatment step (S2), the hot dip galvanizing bath 103 satisfying the formula (1) can be prepared from the hot dip galvanizing bath 103 satisfying the formula (2) by lowering the bath temperature T of the hot dip galvanizing bath 103 after the hot dip galvanizing treatment step (S1) in the coarse bottom dross producing step (S1).

In short, in the method for producing a hot-dip galvanized steel sheet according to the present embodiment, the hot-dip galvanizing bath 103 can be easily switched to the state satisfying the formula (1) or the state satisfying the formula (2) by simply changing the bath temperature T. Therefore, in the present embodiment, the switching between the coarse bottom dross producing step (S1) and the hot dip galvanizing treatment step (S2) can be very easily performed by increasing or decreasing the bath temperature T.

As described above, in the present embodiment, the coarse bottom dross producing step (S1) and the hot-dip galvanizing treatment step (S2) are performed to form a hot-dip galvanized layer on the surface of the steel sheet, thereby producing a hot-dip galvanized steel sheet.

In the present embodiment, the intermediate-sized bottom dross, which is a dross defect, is not easily generated in the hot-dip galvanizing treatment step (S2). As a result, the formation of the dross defect in the hot-dip galvanized steel sheet can be suppressed. Further, the hot dip galvanizing facility 10 can be stopped at a frequency (stop frequency) that is suppressed, and the hot dip galvanizing process (S2) can be performed for a long time.

Further, at the time of shutdown, after bath adjustment of hot dip galvanizing bath 103 and before the hot dip galvanizing treatment step (S2), a coarse bottom dross generating step (S1) is performed. Thus, when the hot-dip galvanizing process (S2) is performed again after the shutdown, the hot-dip galvanizing bath 103 containing coarse bottom dross in advance can be used in the hot-dip galvanizing process (S2).

[ method for producing alloyed Hot-Dip galvanized Steel sheet ]

The hot-dip galvanizing treatment method according to the present embodiment described above can be applied to a method for manufacturing an alloyed hot-dip galvanized steel sheet.

The method for producing an alloyed hot-dip galvanized steel sheet according to the present embodiment includes: a step of manufacturing a hot-dip galvanized steel sheet and an alloying step. In the step of manufacturing the hot-dip galvanized steel sheet, the above-described method of manufacturing a hot-dip galvanized steel sheet is performed. In the alloying step, after passing through the step of producing a hot-dip galvanized steel sheet, the produced hot-dip galvanized steel sheet is subjected to an alloying treatment using an alloying furnace 111 shown in fig. 3. The alloying treatment may be performed by a known method. Through the above manufacturing steps, an alloyed hot-dip galvanized steel sheet can be manufactured.

The method for producing a hot-dip galvanized steel sheet and the method for producing an alloyed hot-dip galvanized steel sheet according to the present embodiment are described in detail above. In the present embodiment, hot dip galvanizing bath 103 containing coarse bottom dross in advance is used at δ ₁ The hot dip galvanizing treatment step (S2) is performed in the nucleus generation area 31. Therefore, during the period of performing the hot dip galvanizing treatment (i.e., during the operation period), the formation and growth of fine bottom dross can be effectively suppressed by the ostwald growth of coarse bottom dross in the hot dip galvanizing bath 103. As a result, during the hot-dip galvanizing process, the formation of intermediate-sized bottom dross, which is a cause of dross defects, can be suppressed, and dross defects in the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet can be suppressed.

Further, in general, in the bottom dross working, when the amount of bottom dross deposited on the bottom of the molten zinc pot 101 increases, the deposited middle-sized bottom dross is rolled up by the accompanying flow, and therefore, dross defects are likely to occur. However, in the case of the present embodiment, coarse bottom dross is deposited on the bottom of the molten zinc pot 101. The coarse bottom dross has a particle size of 300 μm or more and is not easily curled up by the wake due to its mass. Therefore, the coarse bottom dross is less likely to cause the defects in the zinc dross. Therefore, the frequency of the step of removing the bottom dross in the molten zinc pot 101 can be reduced. As a result, the frequency of stopping (stopping) the hot-dip galvanizing facility 10 can be reduced, and productivity can be improved.

Examples

The method for producing a hot-dip galvanized steel sheet according to the present embodiment will be specifically described with reference to the present invention examples and comparative examples. The following examples are merely examples of the method for producing a hot-dip galvanized steel sheet according to the present embodiment. Therefore, the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment is not limited to the following examples.

[ example 1]

[ test in consideration of coarse bottom dross Generation step ]

Hot dip galvanizing baths were prepared in a laboratory simulating a real machine. While measuring the concentration C of free Al in the hot dip galvanizing bath _Al The temperature was maintained at 0.135%, and the bath temperature T was maintained at 455 ℃ for 10 days (240 hours). That is, hot dip galvanizing is performed on gamma ₂ The grain growth region 22 is maintained for 10 days. In this case, the free Fe concentration in the hot dip galvanizing bath was 0.026%. After 10 days of holding, the form (phase and particle size) of the bottom dross in the hot dip galvanizing bath was examined by the following method.

300g of a sample was collected from a region at the center of the depth, the center of the width, and the center of the length of the hot dip galvanizing bath. The collected sample is rapidly cooled and solidified. Measurement samples were collected from the cured test specimens. The sample was ground from the rapidly cooled face to measure 0.5 mm. The surface of the measurement sample that was ground was used as the observation surface. Any 5 fields of view in the observation surface were observed with an optical microscope of 200 times. The area of each field was 250. Mu. M.times.250. Mu.m. In each field, the parent phase (Zn) and the dross can be easily distinguished by contrast. Then, the zinc dross in each visual field was measured.

The phase of the dross was determined for each field by the following method. The chemical composition of each zincilate was analyzed using EPMA. Further, the crystal structure of each zinciferous slag was analyzed using TEM. As a result, the chemical composition of each of the 5 visual fields consisted of 2% by mass of Al, 8% by mass of Fe and 90% by mass of Zn, and the crystal structure thereof was found to beIs a face-centered cubic crystal. Therefore, the dross in the hot dip galvanizing bath was determined to be Γ ₂ And (4) phase zinc slag. Then, Γ for each field of view is determined by the method described above ₂ Particle size of the phase zinc slag. As a result, all F in the 5 fields of view ₂ The average grain size of the phase zinc dross is more than 100 μm. The photographed image in fig. 8 (a) is an example of an image obtained by the scanning electron microscope in example 1. The particles denoted by "Γ 2" in the figures are Γ ₂ And (4) phase zinc slag. The higher the lightness in the image, the higher the Al concentration (see Al (%) on the right in fig. 8).

Through the above tests, the free Al concentration C in the hot dip galvanizing bath of the bath was measured _Al Maintaining the bath temperature T at 455 ℃ at 0.135%, and maintaining the bath temperature T at 455 ℃ for 10 days, the steel sheet is hot-dip galvanized to form a gamma of 100 μm or more in particle size ₂ And (4) phase zinc slag. The concentration C of free Al in the hot dip galvanizing bath during bath preparation _Al Maintained at 0.135%, bath temperature T at 455 deg.C, and holding for 90 days, the result was Γ in 5 fields of view ₂ The grain size of the phase zinc slag is more than 300 mu m.

[ example 2]

[ test assuming Hot Dip galvanizing treatment Process ]

Next, a hot dip galvanizing bath in a laboratory was prepared in the same manner as in example 1. While measuring the concentration C of free Al in the hot dip galvanizing bath _Al The temperature was maintained at 0.135%, and the bath temperature T was maintained at 470 ℃ for 10 days (240 hours). That is, the hot dip galvanizing bath is set to delta ₁ The nuclei were maintained in the nucleus-generating region 31 for 10 days. After 10 days of holding, the form (phase and particle size) of the bottom dross in the hot dip galvanizing bath was examined by the following method. The morphology (phase and particle size) of the bottom dross in the hot dip galvanizing bath was examined by the same method as in example 1. The photographed image of fig. 8 (B) is an example of an image obtained by the scanning electron microscope of example 2. "δ 1" indicated by an arrow in the figure is δ ₁ And (5) phase zinc slag.

As shown in fig. 8, the chemical composition of each of the 5 visual fields of the zinciferous slags is 1% or less of Al, 9% or more of Fe, and 90% or more of Zn by mass%, and the crystal structure is face-centered cubic.Therefore, the zinc dross in the hot dip galvanizing bath of example 2 was regarded as δ ₁ And (5) phase zinc slag. Determination of delta by the method described above ₁ Grain size of the phase zinc slag. As a result, all deltas in 5 fields of view ₁ The grain sizes of the phase zinc slag are far less than 100 mu m. Note that Γ having a particle diameter of 100 μm or more was not confirmed in all 5 fields of view ₂ And (4) phase zinc slag.

The above test results of example 1 and example 2 show agreement with the expected dross from the metastable state diagram of fig. 1. Therefore, it is found that the concentration C of free Al in the hot dip galvanizing bath is appropriately adjusted _Al The particle size of the bottom slag can be adjusted by the bath temperature T (DEG C).

[ example 3]

In addition to examples 1 and 2, alloyed hot-dip galvanized steel sheets were produced by the following method using an actual continuous hot-dip galvanizing facility.

In each test number, during the shutdown period, the "Al concentration C" in the column of "shutdown period" in table 1 was used _Al The free Al concentration C of the hot dip galvanizing bath is maintained as shown in the columns of "column" and "bath temperature T _Al (mass%) and bath temperature T (. Degree. C.). The retention period was 30 days.

After the shutdown period, a hot dip galvanizing process is performed. During the hot dip galvanizing treatment step, the free Al concentration C of the hot dip galvanizing bath was maintained as shown in the column "at run" in Table 1 _Al (mass%) and bath temperature T (. Degree. C.). The retention period was 5 days. The plate throughput of the steel plate during the holding period was the same under each test condition. The steel sheet after the hot dip galvanizing treatment is subjected to a known alloying treatment. The steel sheets used in the respective test numbers are the same in type. The alloying treatment conditions were the same for each test number. Through the above steps, alloyed hot-dip galvanized steel sheets of the respective test numbers were produced.

The free Al concentration C for each test number was determined by the above-mentioned method _Al Measuring with time, and adjusting the free Al concentration C of the hot dip galvanizing bath _Al 。

The surface of the hot-dip galvannealed steel sheet after hot-dip galvanizing was visually observed for the final 2 hours of the holding period, and the dross defect was evaluated according to the following evaluation index.

Specifically, samples were collected from the center of the surface of the galvannealed layer of the galvannealed steel sheet in any width direction. In the surface of the galvannealed layer of the collected sample, a rectangular area of 1m × 1m was set as 1 field of view, and arbitrary 10 fields of view were set as measurement targets. In each visual field, the zinc dross with a particle size of 100 μm or more was visually observed. In the case where the dross having a particle size of 100 μm or more is attached to the alloyed hot-dip galvanized layer, it is considered as a dross defect. The total number of determined dross defects in 10 fields of view was calculated. According to the total number of the zinc slag defects and the total area (10 m) of 10 visual fields ² ) The number of zinc dross defects per unit area (one/10 m) was determined ² ). It is difficult to visually determine whether or not the zinc dross has a particle size of 100 μm or more by using an optical microscope of 100 magnifications.

The evaluation criteria for the zinciferous slag defects are as follows.

Evaluation A: the number of the zinc dross defects per unit area is 0-1/10 m ² 。

Evaluation B: the number of zinc dross defects per unit area is 1-10/10 m ² 。

Evaluation C: the number of the zinc dross defects per unit area is 11/10 m ² The above.

[ evaluation results ]

The evaluation results are shown in table 1.

[ Table 1]

TABLE 1

Note that "F" in Table 1 ₂₁₂₂ "column F showing the corresponding test number ₂₁₂₂ The value is obtained. At "F ₂₃ "column F showing the corresponding test number ₂₃ The value is obtained. At "F ₃₁₃₂ "show the corresponding testF for checking numbers ₃₁₃₂ The value is obtained. The state of the hot dip galvanizing bath of each test number during the shutdown is shown in the column of "area" in the column of "at shutdown" in table 1. For example, in the case of test No. 1, the state of the hot dip galvanizing bath during the shutdown was shown to be Γ ₂ A grain growth region. Similarly, the state of the hot dip galvanizing bath for each test number during operation is shown in the column of "region" in the column of "operation" in table 1. For example, in the case of test No. 1, it was shown that the state of the hot dip galvanizing bath during the operation was δ ₁ A grain growth region.

Referring to Table 1, in test Nos. 3 to 6, 13 to 16, 24 to 26, and 33 to 35, the free Al concentration C of the hot dip galvanizing bath during the shutdown period _Al And the bath temperature T satisfies the formula (1). That is, the state of the hot dip galvanizing bath is Γ ₂ A grain growth region. Furthermore, during operation, the free Al concentration C of the hot dip galvanizing bath _Al And the bath temperature T satisfies the formula (2). That is, the state of the hot dip galvanizing bath is δ ₁ The kernel generates a region. Therefore, in the produced galvannealed steel sheet, no dross defects were observed, and dross defects could be effectively suppressed (evaluation a).

On the other hand, in test Nos. 1 and 2, the free Al concentration C of the hot dip galvanizing bath during the operation period _Al And the bath temperature T does not satisfy the formula (2). Specifically, the state of the hot dip galvanizing bath during the operation is not δ ₁ The kernel generates a region, but δ ₁ A grain growth region. Therefore, a zinc dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 7, the free Al concentration C of the hot dip galvanizing bath during the stop time _Al The bath temperature T does not satisfy the formula (1), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Furthermore, during the operation, the free Al concentration C of the hot dip galvanizing bath _Al And the bath temperature T does not satisfy the formula (2), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Therefore, a dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 10, during the operation, the free Al concentration of the hot dip galvanizing bath was highDegree C _Al Bath temperature T does not satisfy formula (2), and the state of the hot dip galvanizing bath is gamma ₂ A grain growth region. Therefore, a zinc dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 11, the free Al concentration C of the hot dip galvanizing bath during the operation _Al Bath temperature T does not satisfy formula (2), and the state of the hot dip galvanizing bath is gamma ₂ A grain growth region. Therefore, a zinc dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 12, the free Al concentration C of the hot dip galvanizing bath during the operation _Al And the bath temperature T does not satisfy the formula (2), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Therefore, a dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 17, the free Al concentration C of the hot dip galvanizing bath during the operation period _Al And the bath temperature T does not satisfy the formula (2), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Therefore, a zinc dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test Nos. 18 and 19, the free Al concentration C of the hot dip galvanizing bath during the shutdown period _Al And the bath temperature T does not satisfy the formula (1), and the state of the hot dip galvanizing bath is delta ₁ The kernel generates a region. Therefore, in the produced galvannealed steel sheet, a small amount of dross defects were observed (evaluation B).

In test Nos. 20 to 22, the concentration C of free Al in the hot dip galvanizing bath during the operation period _Al Bath temperature T does not satisfy formula (2), and the state of the hot dip galvanizing bath is gamma ₂ A grain growth region. Therefore, a zinc dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 23, the free Al concentration C of the hot dip galvanizing bath during the operation _Al The bath temperature T does not satisfy the formula (2), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Therefore, in the produced galvannealed steel sheet, a small amount of dross defects were observed (evaluation B).

In test No. 27, the free Al concentration C of the hot dip galvanizing bath during the stop _Al The bath temperature T does not satisfy the formula (1), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Furthermore, during operation, the free Al concentration C of the hot dip galvanizing bath _Al The bath temperature T does not satisfy the formula (2), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Therefore, a zinc dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 28, the free Al concentration C of the hot dip galvanizing bath during the stop time _Al The bath temperature T does not satisfy the formula (1), and the state of the hot dip galvanizing bath is delta ₁ The kernel generates a region. Therefore, in the produced galvannealed steel sheet, a small amount of dross defects were observed (evaluation B).

In test Nos. 30 and 31, the free Al concentration C of the hot dip galvanizing bath during the operation _Al Bath temperature T does not satisfy formula (2), and the state of the hot dip galvanizing bath is gamma ₂ A grain growth region. Therefore, a dross defect was observed in the produced galvannealed steel sheet (evaluation C).

In test No. 32, the free Al concentration C of the hot-dip galvanizing bath during the operation period _Al And the bath temperature T does not satisfy the formula (2), and the state of the hot dip galvanizing bath is delta ₁ A grain growth region. Therefore, a zinc dross defect was observed in the produced galvannealed steel sheet (evaluation B).

In test nos. 8, 9 and 29, the bath temperature was constant during the shutdown period and during the operation period, and was 470 ℃. In these test numbers, the free Al concentration C of the hot dip galvanizing bath during the shutdown period _Al And bath temperature T did not satisfy formula (1), but no dross defect was observed in the produced galvannealed steel sheet (evaluation a). On the other hand, as described above, in test nos. 18, 19 and 28, the bath temperature was 470 ℃ or more and constant, since the free Al concentration C of the hot dip galvanizing bath during the shutdown period _Al And bath temperature T do not satisfy formula (1), so that in the produced galvannealed steel sheet, a small amount of dross defects were observed (evaluation)Valence B). Therefore, it is considered that the free Al concentration C of the hot dip galvanizing bath during the shutdown period is more stable to suppress the zinciferous slag defects _Al And bath temperature T satisfying the formula (1), and free Al concentration C during operation _Al And the bath temperature T satisfy the formula (2).

[ example 4]

Test No. 4 (Γ in the shutdown period) of example 3 was examined by the following method ₂ Grain growth region, delta during operation ₁ Nuclear generation region), test number 11 (Γ both during shutdown and during operation) ₂ Grain growth region) and test number 18 (both during shutdown and during run-time δ ₁ Nuclear generation area) of the hot dip galvanizing bath.

After the end of the operation period of test No. 4, test No. 11, and test No. 18, samples (liquid phases) were collected from the liquid surface of each hot dip galvanizing bath in a region from the position of the liquid surface to the position of 300mm depth at the center of the length and the center of the width of each hot dip galvanizing bath. When the depth of the hot dip galvanizing bath is assumed to be D (mm) from the liquid surface of the hot dip galvanizing bath to a position 300mm deep, the depth corresponds to a position from the liquid surface to a position near D/10 in the depth direction.

The collected sample was rapidly cooled by using a copper mold and solidified into a rectangular shape. One of the surfaces of the cured sample was defined as the observation plane. The observation surface was mirror-polished. The viewing surface was defined as a range of 20mm × 20mm in the observation surface after mirror polishing. The particle size and the number of the bottom dross contained in the visual field were measured using a laser microscope. Specifically, a field surface of 20mm × 20mm is divided into 100 fine regions of 2mm × 2 mm. Each fine region was observed with an optical microscope to generate a photographed image (optical image). In the photographic image of the fine region, the contrast between the parent phase (Zn) and the bottom dross is different. Therefore, the photographed image of the fine region is binarized by an appropriate threshold value, and as shown in fig. 7, the interface 150 between the matrix 200 and the bottom ash 100 is clarified. The bottom slag 100 in the fine region is specified, and the maximum length LS of each of the specified bottom slag 100 is obtained by image processing. The obtained maximum length is defined as the particle diameter (μm) of the corresponding bottom ash 100. The bottom ash in all the fine areas is determined, and the particle size of the determined bottom ash is determined. Then, the bottom ash in all the fine regions was classified in a predetermined particle size range. Then, the number of the bottom slag of each stage is determined. And making the number of the bottom slag of each level into a histogram.

The sample collection position is located sufficiently above the coarse bottom dross accumulated on the bottom of the molten zinc pot 101. Therefore, the collected sample does not contain coarse bottom dross accumulated on the bottom of the molten zinc pot 101.

Based on the measured particle diameter and the number of bottom slags, a histogram shown in fig. 9 was prepared.

[ evaluation results ]

Referring to fig. 9, test No. 4 (Γ in the shutdown period) as an example of the present invention ₂ Grain growth region, delta during operation ₁ Nuclear generation region) the total number of bottom dross in the visual field plane was the smallest compared with the other test numbers 11 and 18. The following mechanism is considered to play a role in test No. 4. Coarse bottom dross is generated by performing the coarse bottom dross generation step during the shutdown period. Further, as a result of performing a hot dip galvanizing treatment during the operation period using a hot dip galvanizing bath containing coarse bottom dross, the formation and growth of fine δ 1-phase dross is suppressed by ostwald growth. Therefore, the number of bottom dross particles having a particle size of 100 μm or more and less than 300 μm is minimized, and the total number of bottom dross particles is also minimized. As a result, it was considered that no zinciferous residue defect was observed.

On the other hand, test number 11 (Γ both during shutdown and during operation) ₂ Grain growth region) was the largest in the number of the bottom dross having a grain size of 100 μm or more and less than 300 μm, compared with test No. 4 and test No. 18. Since the number of the bottom dross having a particle size of 100 μm or more and less than 300 μm is large, it is expected that the number of the dross defects is large in test No. 11.

Test No. 18 (both during shutdown and during operation. Delta.) ₁ Nucleus-forming region) was found to have a larger amount of bottom dross with a particle size of 100 μm or more and less than 300 μm than in test No. 4. In particular, the number of the bottom dross having a particle size of 100 μm or more and less than 150 μm is large. Therefore, it is expected that zinc is present in comparison with test No. 4The slag has many defects. In test No. 18, since coarse bottom dross did not exist, the fine δ caused by the ostwald growth of the coarse bottom dross could not be sufficiently suppressed during the operation period ₁ As a result of the formation and growth of the phase zinc dross, it is considered that the number of the bottom dross having a particle size of 100 μm or more and less than 150 μm is increased.

[ example 5]

Based on the above test results, the coarse bottom dross generating step was performed for 30 to 40 days during the shutdown of the continuous hot dip galvanizing facility, and then the step of performing the hot dip galvanizing treatment step for 30 to 40 days was repeated for 1 year using a hot dip galvanizing bath containing coarse bottom dross. At this time, the hot dip galvanizing bath in the coarse bottom dross producing step is adjusted so as to satisfy formula (1) and the hot dip galvanizing bath in the hot dip galvanizing treatment step is adjusted so as to satisfy formula (2) by raising or lowering the bath temperature T. As a result, the frequency of the bottom ash removal step was reduced to 1/3, as compared with the case where the operation was performed while repeating the shutdown period of 30 to 40 days and the operation period of 30 to 40 days for 1 year, the free Al concentration CAl was constantly set to 0.130% in both the shutdown period and the operation period, and the bath temperature was constantly set to 455 ℃.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be modified as appropriate without departing from the scope of the invention.

Description of the reference numerals

10. Hot-dip galvanizing equipment

101. Molten zinc pot

103. Hot dip galvanizing bath

Claims

1. A method for manufacturing a hot-dip galvanized steel sheet, comprising:

a coarse bottom slag generation step of adjusting the free Al concentration C in the hot dip galvanizing bath so as to satisfy the formula (1) _Al And a bath temperature T, thereby generating coarse bottom dross with a particle size of 300 [ mu ] m or more in the hot dip galvanizing bath; and

a hot dip galvanizing treatment step of adjusting the free Al concentration C of the hot dip galvanizing bath after the coarse bottom dross formation step so as to satisfy formula (2) _Al And the bath temperature T, using the free Al concentration C _Al And the hot dip galvanizing bath having the bath temperature T satisfying the formula (2) to perform a hot dip galvanizing treatment to form a hot dip galvanized layer on the steel sheet,

the concentration C of the free Al in the hot dip galvanizing bath in the coarse bottom dross forming step and the hot dip galvanizing treatment step _Al Is set to 0.125 to 0.138 mass%,

466.15×C _Al +385.14≤T≤577.24×C _Al +382.49 (1)

390.91×C _Al +414.20≤T≤485.00 (2)

here, "C" in the formulae (1) and (2) _Al "the free Al concentration C in mass% in the hot dip galvanizing bath _Al 。

2. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein,

and performing the coarse bottom dross generating step on the hot dip galvanizing bath after the hot dip galvanizing treatment step while a machine is stopped for stopping the hot dip galvanizing treatment step.

3. The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein,

4. The method for manufacturing a hot-dip galvanized steel sheet according to claim 2, wherein,

5. The method for producing a hot-dip galvanized steel sheet according to claim 3, further comprising the step of,

repeatedly and alternately performing the coarse bottom dross generating step and the hot dip galvanizing treatment step,

in the case where the coarse bottom dross producing step is performed after the hot-dip galvanizing treatment step, the bath temperature T of the hot-dip galvanizing bath after the hot-dip galvanizing treatment step is lowered in the coarse bottom dross producing step, thereby producing the hot-dip galvanizing bath satisfying formula (1).

6. The method for producing a hot-dip galvanized steel sheet according to claim 4, further comprising the step of,

7. The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 6, wherein,

before the coarse bottom dross producing step, a bottom dross removing step of removing at least a part of the coarse bottom dross in the hot dip galvanizing bath is further provided.

8. A method for manufacturing an alloyed hot-dip galvanized steel sheet, comprising:

a step of manufacturing the hot-dip galvanized steel sheet by performing the method for manufacturing the hot-dip galvanized steel sheet according to any one of claims 1 to 7; and

and an alloying treatment step of alloying the hot-dip galvanized steel sheet.